Handover delay reduction using unlicensed spectrum

ABSTRACT

A method performed by a wireless communication device for reducing handover delay, wherein the wireless communication device is arranged to operate in a cellular communication system and to operate in a cell using unlicensed spectrum. The method includes receiving a downlink, DL, signal from network node operating a neighbouring cell operating in the unlicensed spectrum, wherein the DL signal includes a discovery reference signal, DRS, subframe, storing data associated with the DRS subframe, receiving a handover command from a network node operating a serving cell where the neighbouring cell is a target cell, and performing a random access procedure for handover to the target cell. A device performing the method and a computer program for implementing the method are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Submission Under 35 U.S.C. § 371 for U.S. NationalStage Patent Application of International Application No.:PCT/EP2018/065623, filed Jun. 13, 2018 entitled HANDOVER DELAY REDUCTIONMETHOD, WIRELESS COMMUNICATION DEVICE, AND COMPUTER PROGRAM,” whichclaims priority to U.S. Provisional Application No. 62/520,090, filedJun. 15, 2017, entitled “HANDOVER DELAY REDUCTION METHOD, WIRELESSCOMMUNICATION DEVICE, AND COMPUTER PROGRAM,” the entireties of both ofwhich are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure generally relates to a method performed by awireless communication device for reducing handover delay, such awireless device and a computer program for implementing the method.

Abbreviations Abbreviation Explanation CCA Clear Channel Assessment DCIDownlink Control Information DL Downlink DMRS Demodulation ReferenceSignals eNB, eNodeB evolved NodeB, base station eSSS enhanced SSS TTITransmission-Time Interval UE User Equipment UL Uplink LA LicensedAssisted LAA Licensed Assisted Access DRS Discovery Reference SignalSCell Secondary Cell SRS Sounding Reference Signal SSS SecondarySynchronization Signal LBT Listen-before-talk PDCCH Physical DownlinkControl Channel PSS Primary Synchronization Signal PUSCH Physical UplinkShared Channel PUCCH Physical Uplink Control Channel RRC Radio ResourceControl RRM Radio Resource Management TCS Transmission ConfirmationSignal MF MuLTEfire, MulteFire MFA MuLTEfire Alliance AS Access StratumCN Core Network DRB Data Radio Bearer EPC Evolved Packet Core EPSEvolved Packet System E-UTRA Evolved Universal Terrestrial Radio AccessE-UTRAN Evolved Universal Terrestrial Radio Access Network MME MobilityManagement Entity NAS Non-Access Stratum PDCP Packet Data ConvergenceProtocol RAN Radio Access Network RLC Radio Link Control SIB SystemInformation Block ExtSIB Extended SIB PRACH Physical Random AccessChannel RAR Random Access Response

BACKGROUND

The 3^(rd) Generation Partnership Project, 3GPP, work on“Licensed-Assisted Access” (LAA) intends to allow Long Term Evolution,LTE, equipment to also operate in the unlicensed radio spectrum.Candidate bands for LTE operation in the unlicensed spectrum include 5GHz, 3.5 GHz, etc. The unlicensed spectrum is used as a complement tothe licensed spectrum or allows completely standalone operation.

For the case of unlicensed spectrum used as a complement to the licensedspectrum, devices connect in the licensed spectrum (primary cell, PCell)and use carrier aggregation to benefit from additional transmissioncapacity in the unlicensed spectrum (secondary cell, SCell). Carrieraggregation (CA) framework allows to aggregate two or more carriers withthe condition that at least one carrier (or frequency channel) is in thelicensed spectrum and at least one carrier is in the unlicensedspectrum. In the standalone (or completely unlicensed spectrum) mode ofoperation, one or more carriers are selected solely in the unlicensedspectrum.

Regulatory requirements, however, may not permit transmissions in theunlicensed spectrum without prior channel sensing, transmission powerlimitations or imposed maximum channel occupancy time. Since theunlicensed spectrum must be shared with other radios of similar ordissimilar wireless technologies, a so called listen-before-talk (LBT)method needs to be applied. LBT involves sensing the medium for apre-defined minimum amount of time and backing off if the channel isbusy. Due to the centralized coordination and dependency of terminaldevices on the base-station (eNB) for channel access in LTE operationand imposed LBT regulations, LTE uplink (UL) performance is especiallyhampered. UL transmission is becoming more and more important withuser-centric applications and the need for pushing data to cloud.

Today, the unlicensed 5 GHz spectrum is mainly used by equipmentimplementing the IEEE 802.11 Wireless Local Area Network (WLAN)standard. This standard is known under its marketing brand “Wi-Fi” andallows completely standalone operation in the unlicensed spectrum.Unlike the case in LTE, Wi-Fi terminals can asynchronously access themedium and thus show better UL performance characteristics especially incongested network conditions.

Unlicensed bands offer the possibility for deployment of radio networksby non-traditional operators that do not have access to licensedspectrum, such as e.g. building owners, industrial site andmunicipalities who want to offer a service within the operation theycontrol. Recently, the LTE standard has been evolved to operate inunlicensed bands for the sake of providing mobile broadband usingunlicensed spectrum. The 3GPP based feature of License Assisted Access(LAA) was introduced in Rel. 13, supporting carrier aggregation betweena primary carrier in licensed bands, and one or several secondarycarriers in unlicensed bands. Further evolution of the LAA feature,which only supports DL traffic, was specified within the Rel. 14 featureof enhanced License Assisted Access (eLAA), which added the possibilityto also schedule uplink traffic on the secondary carriers. In parallelto the work within 3GPP Rel. 14, work within the MulteFire Alliance(MFA) aimed to standardize a system that would allow the use ofstandalone primary carriers within unlicensed spectrum. The resultingMulteFire 1.0 standard supports both UL and DL traffic.

Discovery reference signals (DRS) are transmitted periodically to allowfor initial cell detection and channel quality measurements foridle/connected mode mobility. DRS comprises synchronization signals,such as PSS and SSS, other system information, such as informationcontained in a physical broadcast channel (PBCH), common physical shareddata channel (PDSCH) and reference signals, such as cell-specificreference signal (CRS). The DRS transmission window (DTxW) defines aperiodic window during which the eNB attempts DRS transmission. The eNBcan select the length of the DTxW, which can be any integer valuebetween 1-10 ms. The length can e.g. configured depending on the cellload, which will impact the LBT success rate. The DRS periodicity can beset to 40 ms, 80 ms, or 160 ms.

Primary and secondary synchronization signals (PSS, SSS), PBCH, andcommon PDSCH with associated physical downlink control channel (PDCCH)may be provided in a first subframe of a DRS period.

When the DRS is transmitted on subframe (SF) #0-4, the corresponding CRSuses scrambling corresponding to SF #0 and when DRS is transmitted on SF#5-9, the corresponding CRS uses scrambling corresponding to SF #5.Unicast PDSCH can only be multiplexed with DRS if the DRS is transmittedin SF #0 or SF #5, respectively.

Mobility in RRC_Connected state is controlled by the network. The eNBtypically configures UE to measure and report the radio conditions ofthe serving and neighbour cells. Based on the reported radio conditionsand/or network load conditions the eNB initiates the handover process.Alternatively, the eNB may also initiate a blind handover, i.e. withoutwaiting for the measurement reports from the UE. Upon receiving thehandover command from the source cell, the UE performs random accessprocedure towards the target cell to complete the handover process.

A typical cell search procedure for a UE operating in an LTE system istypically performed as follows:

1. RSSI scan involves the UE searching sequentially through thefrequencies in the frequency band and measuring the RSSI. The RSSIvalues are measured at the centre frequency across the interestingbandwidths. The end result is a list of frequencies with the RSSImeasurements. The frequencies with the strongest RSSI values are furtherprocessed.

2. Acquire symbol level synchronization and determine the physical cellidentity of the cell with the PSS and SSS signals.

3. Acquire frame timing to the cell by decoding a master informationblock (MIB) from a physical broadcast channel (PBCH).

4. Receive and decode cell system information.

5. Access the cell, i.e. random access procedure

For random access channel (RACH) procedure in licensed carrieroperation, in order to determine the RACH opportunity (or also known as(P)RACH occasion), i.e. the subframe number in which it can performrandom access, the UE needs to know the frame timing which is providedin the target cell's Secondary Synchronization Signal (SSS). For RACHprocedure in unlicensed carrier operation, i.e. according to MFAspecifications, the DRS can be floating, i.e. PSS/SSS are not fixed toSF #0 or SF #5. Therefore, the subframe offset is provided in the MIB,which is mapped to PBCH. So in contrast to LTE, the UE needs to read MIBbefore it can perform random access.

The LBT procedure leads to uncertainty at the eNodeB (eNB) regardingwhether it will be able to transmit a downlink (DL) subframe(s) or not.This leads to a corresponding uncertainty at the user equipment (UE) asto if it actually has a subframe to decode or not. An analogousuncertainty exists in the UL direction where the eNB is uncertain if theUEs actually transmitted or not.

In the unlicensed radio spectrum, eNB must perform listen before talk(LBT) prior to data transmission on unlicensed band. LBT Category 4 withexponential backoff is a non-aggressive scheme that allows goodcoexistence with Wi-Fi and other unlicensed spectrum users. Thediscovery signal that is transmitted every 40 ms or so is an importantreference signal transmitted to allow the UE to maintain coarsesynchronization with the eNB. The eNB will use a more aggressive LBTmechanism for DRS transmission to ensure that the UE is not starved.Even so, due to the load in the band it cannot be guaranteed that itwill always succeed. Current assumption is, that the start of thediscovery signal is restricted to LTE subframe borders and that thestart of regular data transmissions is restricted to a few fixedpositions within the subframe, including the subframe border.

In unlicensed carrier operation, the handover delay may be long due tothe target cell PBCH unavailability caused by LBT failure, resulting inlonger service interruption and bigger latency for data services. Alsoin the cases where DRS transmission succeeds in the target cell, wherethe DRS periods can vary between 40 to 160 ms, which may be significantin terms of the handover delay. It is therefore a desire to provide anapproach for limiting handover delay for handover to a target cell usingunlicensed spectrum.

The above information disclosed in this Background section is only forenhancement of understanding of the background of the invention andtherefore it may contain information that does not form the prior artthat is already known to a person of ordinary skill in the art.

SUMMARY

The disclosure is based on the inventors' understanding that usableinformation for performing handover may already have been available tothe wireless communication device.

According to a first aspect, there is provided a method performed by awireless communication device for reducing handover delay. The wirelesscommunication device is arranged to operate in a cellular communicationsystem and to operate in a cell using unlicensed spectrum, as well ascells using licensed spectrum. The method comprises receiving adownlink, DL, signal from network node operating a neighbouring celloperating in the unlicensed spectrum, wherein the DL signal comprises adiscovery reference signal, DRS, subframe, storing data associated withthe DRS subframe, receiving a handover command from a network nodeoperating a serving cell where the neighbouring cell is a target cell,and performing a random access procedure for handover to the targetcell.

The performing of the random access procedure may be performed directlyafter the reception of the handover command based on the stored datawithout trying to receive the target cell DRS.

Initiation of the random access procedure may be performed within ahandover interruption time, wherein the handover interruption time maybe calculated considering a limited search time for the DRS of thetarget cell, may be configured by the serving node, or may be apredetermined time.

The performing of the random access procedure may be performed based onthe stored data when the stored data is determined to be valid. Thestored data may be determined to be valid based on any one of age of thestored data, signal quality at reception of the stored data, and targetcell timing drift.

The stored data associated with the DRS subframe may include a physicalbroadcast channel, PBCH, wherein the performing of the random accessprocedure may be based on frame timing associated with the PBCH.

The data associated with the DRS subframe may be stored as raw receiveddata, and the performing of the random access procedure may includedecoding the raw received data. The received raw data may be softcombined with stored raw data, wherein the decoding may be performed forthe soft-combined raw data.

The data associated with the DRS subframe may be achieved by decodingthe received signal and storing the decoded data.

The receiving of the DL signal and storing the data associated with theDRS subframe may comprise a refresh procedure of stored data includingmeasuring quality of a newly received DL signal, comparing the measuredquality with a measured quality of previously stored data, wherein thepreviously stored data is maintained if the measured quality is belowthe quality of the previously stored data, and the age of the previouslystored data is below an ageing time threshold. The previously storeddata may be replaced otherwise by data associated with the DRS subframeof the newly received DL signal. The quality may comprise any one ofsignal-to-noise ratio, SNR, signal-to-interference ratio, SIR,signal-to-interference-and-noise ratio, SINR, reference signal receivedpower, RSRP, and reference signal received quality, RSRQ. The ageingtime threshold may be calculated based on estimated time drift inrelation to the target node.

The handover may be from a cell operating in licensed or unlicensedspectrum.

According to a second aspect, there is provided a wireless communicationdevice arranged to operate in a cellular communication system and tooperate in a cell using unlicensed spectrum, as well as cells usinglicensed spectrum. The wireless communication device comprises areceiver arranged to receive a downlink, DL, signal from network nodeoperating a neighbouring cell operating in the unlicensed spectrum,wherein the DL signal comprises a discovery reference signal, DRS,subframe, and a memory arranged to store data associated with the DRSsubframe. The wireless communication device is arranged to, uponreceiving a handover command on a handover from a network node operatinga serving cell to a target cell, where the neighbouring cell is thetarget cell, perform a random access procedure for handover to thetarget cell.

The device may be arranged to perform the random access proceduredirectly based on the stored data without trying to receive the targetcell DRS after the reception of the handover command.

The device may be arranged to initiate the random access procedurewithin a handover interruption time, wherein the handover interruptiontime may be calculated considering a limited search time for the DRS ofthe target cell, may be configured by the serving node, or may be apredetermined time.

The random access procedure may be performed based on the stored datawhen the stored data is determined to be valid. The stored data may bedetermined to be valid based on any one of age of the stored data,signal quality at reception of the stored data, and target cell timingdrift.

The stored data associated with the DRS subframe may include a physicalbroadcast channel, PBCH, wherein the performing of the random accessprocedure may be based on frame timing associated with the PBCH.

The data associated with the DRS subframe may be stored as raw receiveddata, and the device may be arranged to decode the raw received datawhen performing the random access procedure. The device may be arrangedto soft combine received raw data with stored raw data, wherein thedecoding may be performed for the soft-combined raw data.

The data associated with the DRS subframe may be achieved by decodingthe received signal and storing the decoded data.

The device may be arranged to, upon performing the receiving of the DLsignal and storing the data associated with the DRS subframe, refreshstored data by measuring quality of a newly received DL signal,comparing the measured quality with quality of previously stored data,wherein the previously stored data is maintained if the measured qualityis below the quality of the previously stored data, and the age of thepreviously stored data is below an ageing time threshold, and replacedotherwise by data associated with the DRS subframe of the newly receivedDL signal. The quality may comprise any one of signal-to-noise ratio,SNR, signal-to-interference ratio, SIR, signal-to-interference-and-noiseratio, SINR, reference signal received power, RSRP, and reference signalreceived quality, RSRQ. The ageing time threshold may be calculatedbased on estimated time drift in relation to the target node.

The handover may be from a cell operating in licensed or unlicensedspectrum.

According to a third aspect, there is provided a computer programcomprising instructions which, when executed on a processor of awireless communication device, causes the wireless communication deviceto perform the method according to the first aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

The above, as well as additional objects, features and advantages of thepresent disclosure, will be better understood through the followingillustrative and non-limiting detailed description of preferredembodiments of the present disclosure, with reference to the appendeddrawings.

FIG. 1 is a flow chart illustrating a method of a wireless communicationdevice for reducing handover delay according to an embodiment.

FIG. 2 is a flow chart illustrating a procedure for refreshing storeddata according to an embodiment.

FIG. 3 is a block diagram schematically illustrating a wirelesscommunication device according to an embodiment.

FIG. 4 schematically illustrates a computer-readable medium and aprocessing device.

FIG. 5 is a signal scheme illustrating a handover procedure according toan example.

FIG. 6 illustrates arranging of LTE downlink physical resources.

FIG. 7 illustrates LTE time-domain structure.

FIG. 8 illustrates an LTE downlink subframe.

FIG. 9 illustrates an LTE uplink subframe.

DETAILED DESCRIPTION

LTE uses OFDM in the downlink and DFT-spread OFDM (also referred to assingle-carrier FDMA) in the uplink. The basic LTE downlink physicalresource can thus be seen as a time-frequency grid as illustrated inFIG. 6, where each resource element corresponds to one OFDM subcarrierduring one OFDM symbol interval. The uplink subframe has the samesubcarrier spacing as the downlink and the same number of SC-FDMAsymbols in the time domain as OFDM symbols in the downlink.

In the time domain, LTE downlink transmissions are organized into radioframes of 10 ms, each radio frame consisting of ten equally-sizedsubframes of length T_(subframe)=1 ms as shown in FIG. 7. Each subframecomprises two slots of duration 0.5 ms each, and the slot numberingwithin a frame range from 0 to 19. For normal cyclic prefix, onesubframe consists of 14 OFDM symbols. The duration of each symbol isapproximately 71.4 μs.

Furthermore, the resource allocation in LTE is typically described interms of resource blocks, where a resource block corresponds to one slot(0.5 ms) in the time domain and 12 contiguous subcarriers in thefrequency domain. A pair of two adjacent resource blocks in timedirection (1.0 ms) is known as a resource block pair. Resource blocksare numbered in the frequency domain, starting with 0 from one end ofthe system bandwidth.

Downlink transmissions are dynamically scheduled, i.e., in each subframethe base station transmits control information about which terminalsdata is transmitted to and upon which resource blocks the data istransmitted, in the current downlink subframe. This control signallingis typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in eachsubframe and the number n=1, 2, 3 or 4 is known as the Control FormatIndicator (CFI). The downlink subframe also contains common referencesymbols, which are known to the receiver and used for coherentdemodulation of e.g. the control information. A downlink system withCFI=3 OFDM symbols as control is illustrated in FIG. 8. The referencesymbols shown in FIG. 8 are the cell specific reference symbols (CRS)and are used to support multiple functions including fine time andfrequency synchronization and channel estimation for certaintransmission modes.

Uplink transmissions are dynamically scheduled, i.e., in each downlinksubframe the base station transmits control information about whichterminals should transmit data to the eNB in subsequent subframes, andupon which resource blocks the data is transmitted. The uplink resourcegrid is comprised of data and uplink control information in the PUSCH,uplink control information in the PUCCH, and various reference signalssuch as demodulation reference signals (DMRS) and sounding referencesignals (SRS). DMRS are used for coherent demodulation of PUSCH andPUCCH data, whereas SRS is not associated with any data or controlinformation but is generally used to estimate the uplink channel qualityfor purposes of frequency-selective scheduling. An example uplinksubframe is shown in FIG. 9. Note that UL DMRS and SRS aretime-multiplexed into the UL subframe, and SRS are always transmitted inthe last symbol of a normal UL subframe. The PUSCH DMRS is transmittedonce every slot for subframes with normal cyclic prefix, and is locatedin the fourth and eleventh SC-FDMA symbols.

From LTE Rel-11 onwards, DL or UL resource assignments can also bescheduled on the enhanced Physical Downlink Control Channel (EPDCCH).For Rel-8 to Rel-10 only the Physical Downlink Control Channel (PDCCH)is available. Resource grants are UE specific and are indicated byscrambling the DCI Cyclic Redundancy Check (CRC) with the UE-specificC-RNTI identifier. A unique C-RNTI is assigned by a cell to every UEassociated with it, and can take values in the range 0001-FFF3 inhexadecimal format. A UE uses the same C-RNTI on all serving cells.

FIG. 1 is a flow chart illustrating a method 100 of a wirelesscommunication device for reducing handover delay according to anembodiment. The flow chart illustrates not only actions specific for thedisclosed approach, but also for legacy actions to put the suggestedapproach into context. These legacy actions are illustrated withdot-dash lined boxes. Furthermore, optional features and/or featuresdemonstrated in greater detail with reference to other figures areillustrated with dashed lines.

The wireless communication device receives 102 a DL signal from aneighbouring node, i.e. a potential target node for handover. Thequality of the reception is measured 103 and a measurement report istransmitted 105 to a serving node of the wireless communication device.Data associated with a discovery reference signal (DRS) subframe isstored 104, optionally together with reception quality data e.g. asmetadata. The stored data may be raw data as received, or demodulatedand/or extracted data related to the information of interest, whichinformation will be elucidated below. An advantage of the formeralternative is that energy and/or processing power is not initiallyspent on processing the received signal, possibly with a cost ofnon-negligible memory resources to store the raw data. An advantage ofthe latter alternative is that the desired information is instantlyavailable when use of it is called upon, and storage space may bereduced compared with the former alternative, while the cost is thatenergy and/or processing power may be spent on decoding and/orextracting information which maybe never is used.

Optionally, a refresh procedure 107 is used where the data associatedwith the DRS subframe is updated where suitable. An example of such arefresh procedure is demonstrated with reference to FIG. 2. In short,the refresh procedure is for keeping at least a good enough set of data,where possible, for improving mobility, i.e. limiting handover delay.This involves further received DL signals and evaluation of these, e.g.in view of signal quality and/or age of the information, in view of thealready stored information.

When a handover command is received 108 from the serving network node,the wireless communication device performs 110 a random access proceduretowards the target node. To be able to do this, the wirelesscommunication device needs to know for example frame timing. Thisinformation may be given in a periodically provided DL signal from thetarget node. However, as discussed above, this may not be provided whenthe target node uses unlicensed spectrum, i.e. because of long DRScycles and/or inability to do such DL transmissions since the channel isnot clear. Thus, in the suggested approach, the performing 110 of therandom access procedure can use the stored data to be able to proceedwith the handover process although no DL transmissions from the targetnode is available after the reception of the handover command.Furthermore and optionally, although such DL transmissions areavailable, quality may be improved by comparing the newly received DLtransmissions and their data with the stored data and selecting the onewhich provides the best information quality. In summary, the wirelesscommunication device is able to start and proceed with the handoverprocess swiftly after the handover command is received from the servingnode. For example, the wireless communication device may initiate randomaccess procedure transmissions with the target node in the nextavailable random access occasion without the need to receive DRS of thetarget cell if uplink LBT succeeds. The wireless communication devicemay have a reduced target time period or time limit for how long afterthe handover command is received that the handover procedure iscompleted in the target cell, without the need to consider the requiredtime spent on searching for DRS of the target cell. A time limit, i.e.handover interruption time, which is a part of the service interruptiontime, may for example be 100 ms, 200 ms or more depending on the DRStransmission periodicity, and may be configured as a timer from theserving node, or as a requirement in the verification process. The timelimit may be based on a delay requirement defined in specifications forthe system, e.g. a RAN4 requirement, or be given from a specified RRCtimer of the specifications, e.g. T304 as specified in e.g. 3GPP TS36.331.

The handover process is then completed 111, i.e. the previous servingcell makes a UE context release and previous target cell now becomes thenew serving cell. The procedure 100 then continues by making newreceptions 102 and measurements 103 on new neighbouring cells, etc.

FIG. 5 is a signal scheme illustrating a handover procedure according toan example. The interacting elements are the wireless communicationdevice (WCD), the source node (SN) and the target node (TN). The WCDreceives the DRS associated data from the TN and performs channelquality measurements so that the WCD can send a measurement report tothe SN. The WCD also stores data associated with the DRS subframe, asdemonstrated above. The SN makes a handover decision and transmits ahandover request to the TN, and the SN sends a handover command to theWCD upon receiving a handover request acknowledgement from the TN. TheWCD is thus about to initiate the handover procedures and needs to knowframe timing of the TN. DRS signal may be subject to LBT failure andthus WCD may not access the next available DRS signal from TN, which maylead to an increased service interruption time. By applying the approachdemonstrated above, the WCD is able to acquire the frame timing anyway,i.e. from the stored data, and can commence the handover procedure, i.e.making random access transmissions with the TN. The TN can thus make apath switch with a mobility management entity (MME), provide a UEcontext release to the SN and take over as a serving node for the WCD.The SN can release UE context, and the handover is completed.

Returning to FIG. 1, the storing 104 may in one example demonstratedabove comprise storing raw received data. This may comprise storing thesignal corresponding to the centre six resource blocks of a subframecontaining the latest neighbour cell measurement samples which triggereda measurement report. From this, PBCH is decoded after receiving 108 thehandover command. For the case there is more than one potential targetcell, and thus more than one stored set of data, the one correspondingto a PCI of the handover command is used for the decoding. The PBCHprovides system information, i.e. MIB, from which frame timing isavailable.

One option for the case where raw received data is stored is that softcombining with newly received data may be performed for improving thepossibilities for proper decoding of PBCH.

Quality of the received signal with PBCH is determined. The quality maybe seen as any one, or combination, of RSRP, RSRQ, SIR, SNR, SINR, orother recognised signal quality measure. Information about the qualityof stored data, i.e. estimated quality at reception of the stored data,may be saved together with the stored data, e.g. as metadata.Reasonably, only data with quality over some threshold, i.e. usable, isstored.

The storing 104 may be performed for any or all signals holding PBCH,but to limit processing only data associated with DRS subframe whichhave been subject to a measurement report transmission associated tohandover. For example, it may be a triggered measurement event such asA3, A4, or A5. Considering the quality reasoning above, data from areception triggering a handover measurement report inherently have areasonable signal quality; Otherwise it would not be a subject forhandover.

Subframe offset is a relative number to subframe #0 or #5 in a radioframe. A timestamp may thus be stored associated to each DRS subframefor deriving the cell timing at a later stage.

Different types and categories of wireless communication devices havedifferent capabilities in sense of ability to perform concurrentreception and processing. This may call for different preferred variantsof the above demonstrated approach. For example, where the wirelesscommunication device has capabilities for decoding of neighbour cells'MIB while connected to and performing actions with the serving cell,decoding and extraction of the desired data may be performed as thesignals are received and measured. In such case, raw data is notnecessary to be stored, but may be so considering the soft combiningfeature discussed above. The extracted data is stored together with forexample one or more of subframe offset, quality information, timestamp,etc. For wireless communication devices not having the capability todecode and extract the desired data from the neighbouring cells, it maybe necessary to store raw IQ data received from the neighbouring cells,wherein the desired data is decoded and extracted when handover commandis received.

If the target node is not synchronized with the source node, the targetnode timing may drift away. Stored data may thus be ageing, and thus,the stored data needs to be dropped after some time since the obtainedframe timing may not be valid anymore. Then new data needs to be storedwhen data of proper quality is available. A timer may be provided toguard the freshness of the stored data. The timer or ageing time limitsmay be determined based on estimated time drift in relation to thetarget node, which in turn may be estimated based on one or more ofestimated timing accuracy of the wireless communication device and/orserving cell, estimated timing accuracy of target cell, and timingrequirements specified for the communication system.

Further, apart from the timer the UE could additionally overwrite oldvalues with DRS data stored for the latest measurement or if the RSRPmeasurement is better than the previous. The latter requires that themeasurement result is also stored together with the DRS subframe data,as has been discussed above. An aggregate evaluation of ageing andquality of the stored data may also be provided.

FIG. 2 is a flow chart illustrating a procedure 200 for refreshingstored data according to an embodiment. A DL signal from a neighbouringnode is received 201, and the quality of the received signal is measured202. These actions are recognised from the description of FIG. 1 and maybe the same, but may also be separate actions. It is here assumed thatdata has previously been stored in the memory from a previous reception,and that the approach is up and running, as for example demonstratedwith reference to FIG. 1, or considering the approach starting from step204 of storing the data associated with the DRS subframe.

The quality of the newly received signal is compared 203 with storedquality of the stored data. If the quality of the new data is betterthan the stored one, considering any of the quality metrics demonstratedabove, the new data associated with the DRS subframe is stored 204. Theway and format of storing may be any of the alternatives demonstratedabove, i.e. raw data or extracted data, and with different amounts ofadditional data such as quality and timestamp. An age timer is started205 for enabling keeping track of age of the stored data. Here, thetimer may be a physical timer or any mechanism providing the similareffect, e.g. metadata with a timestamp for the stored data. If thequality of the new data is not better than the stored one, the procedure200 short-cuts the storing 204 and timing 205 steps.

The age of the stored data is tested 206, i.e. it is checked whether theage timer has expired, the age of a timestamp associated with the storeddata is checked against a time reference, or any similar determinationof whether the stored data is still valid in sense of ageing. If thestored data is too old, it is set 207 as not valid, which can be made indifferent ways. The stored data can for example be deleted, the qualitycan be set to a zero value, a non-valid flag can be set, etc. If the ageis OK, the stored data is kept. The refresh procedure 200 continuouslykeeps the stored data in shape.

The illustration of the refresh procedure 200 should be construed forunderstanding the principles, and not as a direct and onlyimplementation of the procedure 200. A reasonable way of implementingthe procedure 200 is as a real-time mechanism comprising receivingobject, a measurement object, a quality object and a timing objectmutually interacting whenever new data, evaluations, or updates areavailable. Other ways of organising the objects are of course equallyfeasible.

FIG. 3 is a block diagram schematically illustrating a wirelesscommunication device 300, e.g. user equipment, UE, according to anembodiment. The wireless communication device 300 comprises an antennaarrangement 302, a receiver 304 connected to the antenna arrangement302, a transmitter 306 connected to the antenna arrangement 302, aprocessing element 308 which may comprise one or more circuits, one ormore input interfaces 310 and one or more output interfaces 312. Theinterfaces 310, 312 can be user interfaces and/or signal interfaces,e.g. electrical or optical. The wireless communication device 300 isarranged to operate in a cellular communication network. In particular,the wireless communication device 300 comprises a memory 314 enablingthe wireless communication device 300 to be arranged to perform theembodiments demonstrated with reference to FIGS. 1 and 2. The access tothe memory may be directly to and from the receiver 304, or be via orunder control of the processing element 308. The memory 314 may bearranged to store raw data received by the receiver 304 for decodingupon need by the receiver, or the memory 314 may be arranged to storedecoded data associated with the DRS subframe. Thus, the wirelesscommunication device 300 is capable of efficient mobility in sense oflimiting handover delay. The processing element 308 can also fulfill amultitude of tasks, ranging from signal processing to enable receptionand transmission since it is connected to the receiver 304 andtransmitter 306, executing applications, controlling the interfaces 310,312, etc.

The methods according to the present disclosure is suitable forimplementation with aid of processing means, such as computers and/orprocessors, especially for the case where the processing element 308demonstrated above comprises a processor handling mobility. Therefore,there is provided computer programs, comprising instructions arranged tocause the processing means, processor, or computer to perform the stepsof any of the methods according to any of the embodiments described withreference to FIGS. 1 and 2. The computer programs preferably compriseprogram code which is stored on a computer readable medium 400, asillustrated in FIG. 4, which can be loaded and executed by a processingmeans, processor, or computer 402 to cause it to perform the methods,respectively, according to embodiments of the present disclosure,preferably as any of the embodiments described with reference to FIGS. 1and 2. The computer 402 and computer program product 400 can be arrangedto execute the program code sequentially where actions of the any of themethods are performed stepwise, but may as well be performed on areal-time basis, for example as discussed above for the refreshprocedure 200. The processing means, processor, or computer 402 ispreferably what normally is referred to as an embedded system. Thus, thedepicted computer readable medium 400 and computer 402 in FIG. 4 shouldbe construed to be for illustrative purposes only to provideunderstanding of the principle, and not to be construed as any directillustration of the elements.

The invention claimed is:
 1. A method performed by a wirelesscommunication device for reducing handover delay, the wirelesscommunication device being arranged to operate in a cellularcommunication system and to operate in a cell using unlicensed spectrum,the method comprising: receiving a downlink (DL) signal from a firstnetwork node operating a neighboring cell operating in the unlicensedspectrum, the DL signal comprising a discovery reference signal (DRS)subframe; storing data associated with the DRS subframe; receiving ahandover command from a second network node operating a serving cellwhere the neighboring cell is a target cell; and performing a randomaccess procedure for handover of the wireless communication device fromthe serving cell to the target cell using the stored data, performingthe random access procedure being initiated during a handoverinterruption time that is limited by a periodicity of transmission ofDRS subframes.
 2. The method of claim 1, wherein the performing of therandom access procedure is performed directly after the reception of thehandover command based on the stored data without trying to receive thetarget cell DRS.
 3. The method of claim 1, wherein the performing of therandom access procedure is performed based on the stored data when thestored data is determined to be valid.
 4. The method of claim 3, whereinthe stored data is determined to be valid based on any one of: age ofthe stored data; signal quality at reception of the stored data; andtarget cell timing drift.
 5. The method of claim 1, wherein the storeddata associated with the DRS subframe includes a physical broadcastchannel (PBCH) wherein the performing of the random access procedure isbased on frame timing associated with the PBCH.
 6. The method of claim1, wherein the stored data associated with the DRS subframe includes asystem information block (SIB) received via a physical downlink sharedchannel, PDSCH.
 7. The method of any one of claim 1, wherein the dataassociated with the DRS subframe is stored as raw received data, and theperforming of the random access procedure includes decoding the rawreceived data.
 8. The method of claim 7, wherein received raw data issoft combined with stored raw data, wherein the decoding is performedfor the soft-combined raw data.
 9. The method of claim 1, wherein thedata associated with the DRS subframe is achieved by decoding thereceived DL signal and storing the decoded data.
 10. The method of claim1, wherein the receiving of the DL signal and storing the dataassociated with the DRS subframe comprise a refresh procedure of storeddata including: measuring quality of a newly received DL signal;comparing the measured quality with a measured quality of previouslystored data, wherein the previously stored data is: maintained if: themeasured quality is below the quality of the previously stored data; andan age of the previously stored data is below an ageing time threshold;and replaced otherwise by data associated with the DRS subframe of thenewly received DL signal.
 11. The method of claim 10, wherein thequality comprises any one of: signal-to-noise ratio (SNR);signal-to-interference ratio (SIR); signal-to-interference-and-noiseratio (SINR); reference signal received power (RSRP); and referencesignal received quality (RSRQ).
 12. The method of claim 10, wherein theageing time threshold is calculated based on estimated time drift inrelation to the target node.
 13. The method of claim 1, wherein thehandover is from a cell operating in licensed or unlicensed spectrum.14. A wireless communication device arranged to operate in a cellularcommunication system and to operate in a cell using unlicensed spectrum,the wireless communication device comprising: a receiver configured toreceive a downlink (DL) signal from a first network node operating aneighboring cell operating in the unlicensed spectrum, the DL signalcomprising a discovery reference signal (DRS) subframe; and a memoryarranged to store data associated with the DRS subframe, the wirelesscommunication device being configured to, upon receiving a handovercommand on a handover from a second network node operating a servingcell to a target cell, where the neighboring cell is the target cell,perform a random access procedure for handover to the target cell usingthe stored data, performing the random access procedure being initiatedduring a handover interruption time that is limited by a periodicity oftransmission of DRS subframes.
 15. The wireless communication device ofclaim 14, wherein the wireless communication device is configured toperform the random access procedure directly based on the stored datawithout trying to receive the target cell DRS after the reception of thehandover command.
 16. The wireless communication device of claim 14,wherein the random access procedure is performed based on the storeddata when the stored data is determined to be valid.
 17. The wirelesscommunication device of claim 16, wherein the stored data is determinedto be valid based on any one of: age of the stored data; signal qualityat reception of the stored data; and target cell timing drift.
 18. Thewireless communication device of claim 14, wherein the stored dataassociated with the DRS subframe includes a physical broadcast channel(PBCH) wherein the performing of the random access procedure is based onframe timing associated with the PBCH.
 19. The wireless communicationdevice of claim 14, wherein the stored data associated with the DRSsubframe includes a system information block, SIB, received via aphysical downlink shared channel, PDSCH.
 20. The wireless communicationdevice of claim 14, wherein the data associated with the DRS subframe isstored as raw received data, and the wireless communication device isconfigured to decode the raw received data when performing the randomaccess procedure.
 21. The wireless communication device of claim 20,wherein the wireless communication device is configured to soft combinereceived raw data with stored raw data, wherein the decoding isperformed for the soft-combined raw data.
 22. The wireless communicationdevice of claim 14, wherein the data associated with the DRS subframe isachieved by decoding the received DL signal and storing the decodeddata.
 23. The wireless communication device of claim 14, wherein thewireless communication device is configured to, upon performing thereceiving of the DL signal and storing the data associated with the DRSsubframe, refresh stored data by: measuring quality of a newly receivedDL signal; comparing the measured quality with quality of previouslystored data, wherein the previously stored data is: maintained if: themeasured quality is below the quality of the previously stored data; andan age of the previously stored data is below an ageing time threshold;and replaced otherwise by data associated with the DRS subframe of thenewly received DL signal.
 24. The wireless communication device of claim23, wherein the quality comprises any one of: signal-to-noise ratio(SNR); signal-to-interference ratio (SIR);signal-to-interference-and-noise ratio (SINR); reference signal receivedpower (RSRP); and reference signal received quality (RSRQ).
 25. Thewireless communication device of claim 23, wherein the ageing timethreshold is calculated based on estimated time drift in relation to thesecond node.
 26. The wireless communication device of claim 14, whereinthe handover is from a cell operating in licensed or unlicensedspectrum.
 27. A non-transitory computer readable medium storing acomputer program comprising instructions which, when executed on aprocessor of a wireless communication device, causes the wirelesscommunication device to perform reducing handover delay, the wirelesscommunication device being arranged to operate in a cellularcommunication system and to operate in a cell using unlicensed spectrum,comprising: receiving a downlink (DL) signal from a first network nodeoperating a neighboring cell operating in the unlicensed spectrum, theDL signal comprising a discovery reference signal (DRS) subframe;storing data associated with the DRS subframe; receiving a handovercommand from a second network node operating a serving cell where theneighboring cell is a target cell; and performing a random accessprocedure for handover to the target cell using the stored data,performing the random access procedure being initiated during a handoverinterruption time that is limited by a periodicity of transmission ofDRS subframes.